1. Field of the Invention:
The present invention relates to novel cerium based polishing compositions, a process for the preparation thereof, and to the use of same in the polishing of glass, ceramics and other vitreous type materials.
2. Description of the Prior Art:
Among the polishing compositions currently in use in the glass industry, those based on the rare earths, and in particular cerium, are generally the most effective and efficient. Different processes too are known for the production of such compositions. Thus, it has been proposed, in German Pat. No. 2,426,110, to precipitate an aqueous solution of a rare earth sulfate with sodium hydroxide in the presence of molochite, and to then filter, wash, dry and calcine the precipitate to obtain the desired polishing composition. Such a process does not permit the obtainment of polishing compositions having completely satisfactory efficacy, particularly in view of their heterogeneity, undefined structure and lack of reproducibility of the final products. These disadvantages originate specifically from the operating conditions of the process, wherein the concentration of the reagents changes over the course of the reaction and it is thus impossible to obtain defined products in a reproducible fashion; they are also occasioned by the existence of foreign compounds (molochite) and the presence of sulfates, the precipitation of which by sodium hydroxide yields mixtures of complex molecules, such as double sulfates, hydroxysulfates and hydroxides, the amount and nature of which may vary over the course of the reaction.
It has also been proposed, Chemical Abstracts, 80, 51688 (1974), to produce cerium based polishing compositions by precipitating ceric hydroxide with ammonia from a solution of a rare earth nitrate, wherein the cerium has been previously oxidized. Such a process mandates a supplementary purification step by recrystallization with the ceri-ammoniacal nitrate and taking up the calcined oxide in a dilute acid. The final products, moreover, do not have a satisfactory polishing efficacy. Furthermore, such final compositions also do not have satisfactory homogeneity or structural characteristics, and they are not reproducible.
In U.S. Pat. No. 3,768,989 a process is described for the preparation of polishing compositions by the formation of a precipitate of a rare earth carbonate-wollastonite, separation of the precipitate and the calcination thereof. And in U.S. Pat. No. 3,262,766 a process is described for preparing polishing compositions by treating solutions of commercially available rare earth carbonates with either fluosilicic or hydrofluosilicic acid. Polishing compositions prepared by the processes described in either of these two U.S. patents, however, have disadvantages akin to those described above for the compositions prepared according to the teachings of the '110 German patent.
To eliminate the disadvantages of the aforementioned processes, a novel process has been proposed in French Pat. No. 2,472,601, assigned to the assignee hereof, in accordance with which:
(a) a solution of a cerium salt is continuously mixed with a basic solution and a solution of at least one acid and/or salt, the anion or anions of which are capable of forming insoluble rare earth compounds, with the number of equivalents of base being greater than or equal to the number of equivalents of cerium and the pH of the reaction medium being higher than 6;
(b) the resultant precipitate is filtered;
(c) dried; and
(d) thence calcined.
This process provides polishing compositions having good homogeneity and reproducibility, good stability upon suspension in water and good polishing efficacy. However, the immediately aforedescribed process is not entirely satisfactory, since the anion typically employed is fluoride which gives rise to those problems necessarily associated with the use of fluorides, in particular pollution which may be caused by the effluent therefrom, whether in liquid or gaseous state.